Heat shock protein 90 (Hsp90) is an abundant, ubiquitously expressed member of a multiprotein chaperone complex that mediates the stabilization and maturation of proteins, including several that, when genetically altered, are important for oncogenesis. (Drysdale M J, et al. (2006) Curr. Opin. Drug Discov. Devel. 9(4), 483-495) (Whitesell L, et al. (2005) Nat. Rev. Cancer 5(10), 761-772) To date more than 100 “client” proteins of Hsp90 have been identified, including EGF-R, MET, Raf-1 kinase, AKT, Bcr-abl, mutant p53 and CDK4. (Zhang H, et al. (2004) J. Mol. Med., 82, 488-499) (Xu W, et al. (2001) J. Biol. Chem., 276, 3702-3708)
Hsp90 chaperones, which possess a conserved ATP-binding site at their N-terminal domain (Chene, 2002) belong to a small ATPase sub-family known as the DNA Gyrase, Hsp90, Histidine Kinase and MutL (GHKL) sub-family (Dutta and Inouye, 2000). The chaperoning (folding) activity of Hsp90 depends on its ATPase activity, which is weak for the isolated enzyme. However, it has been shown that the ATPase activity of Hsp90 is enhanced upon its association with proteins known as co-chaperones (Kamal et al., 2003). Therefore, in vivo, Hsp90 proteins work as subunits of large, dynamic protein complexes. Among the stress proteins, Hsp90 is unique because it is not required for the biogenesis of most polypeptides (Nathan et al., 1997). Its cellular targets, also called client proteins, are conformationally labile signal transducers that play a critical role in growth control, cell survival and tissue development (Pratt and Toft, 2003). Inhibition of its intrinsic ATPase activity of Hsp90 disrupts the Hsp90-client protein interaction resulting in their degradation via the ubiquitin proteasome pathway.
The Hsp90 family of chaperones is comprised of four members: Hsp90α and Hsp90β both located in the cytosol, GRP94 in the endoplasmic reticulum, and TRAP1 in the mitochondria (Csermely et al., 1998). Hsp90 is the most abundant cellular chaperone, constituting about 1%-2% of total protein (Jakob and Buchner, 1994). Among the stress proteins, Hsp90 is unique because it is not required for the biogenesis of most polypeptides (Nathan et al., 1997).
Hsp90 is essential for eukaryotic cell survival and is overexpressed in many tumors. In tumor cells, Hsp90 allows oncogenic mutant proteins to maintain or gain function while enabling cancer cells to tolerate the imbalance their presence creates. Hsp90 inhibition hinders these pro-oncogenic effects and leads to significant anti-tumor activity in vivo. (Maloney A, et al. (2003) Curr. Cancer Drug Targets, 3, 331-341) A subset of Hsp90 proteins, such as Raf, AKT, phospho-AKT and CDK4 are oncogenic signaling molecules critically involved in cell growth, differentiation and apoptosis, processes which are important in cancer cells. The degradation of one or multiple oncoproteins is believed to produce the anti-tumor effects observed with Hsp90 inhibitors.
Due to its involvement in regulating a number of signaling pathways that are crucial in driving the phenotype of a tumor, and the discovery that certain bioactive natural products exert their effects via Hsp90 activity, the molecular chaperone Hsp90 is valued as a target for anticancer drug development (Neckers et al., 1999). Many known Hsp90 inhibitors exist for cancer therapy. Among the growing number of therapeutic candidate Hsp90 inhibitors, the benzoquinone ansamycin analogs of geldanamycin, 17-allylamino-17-demethoxygeldanamycin (17-AAG) and 17-dimethylamino-geldanamycin (17-DMAG), are Hsp90 inhibitors that are the furthest advanced in clinial studies. (Solit D B, et al. (2007) Clin. Cancer Res., 13(6), 1775-1782) (Hollingshead M, et al. (2005) Cancer Chemother Pharmacol., 56(2), 115-25) A recently described Hsp90 inhibitor, NVP-AUY922, may hold greater promise via its improved pharmaceutical and pharmacological properties as demonstrated by more potent anti-tumor activity in a preclinical model of human breast cancer. (Jensen M R, et al. (2008) Breast Cancer Res., 10(2), R33) (Eccles S, et al. (2008) Cancer Res, 68(8), 2850-2860) In a panel of representative cancer cell lines, NVP-AUY992 inhibited cell proliferation between 3.6- and 300-fold more effectively than 17-AAG.
Known clinical pharmacodynamic biomarkers of Hsp90 inhibition have proven useful, but many (e.g., measurement of Hsp70 induction) require invasive and laborious procedures such as analyses of nucleated blood cell and/or tumor biopsies. As such, there is a real need to identify convenient, circulating pharmacodynamic markers that can accurately and conveniently reflect the in vivo effect of Hsp90 inhibition in tumors. As described herein, the cytokine IL-8 has been identified as a potential secreted biomarker of tumor cell pharmacodynamic response to HSp90 inhibitors both in vitro and in vivo.